Hand Soldering Temperature Control
Hand soldering is used for the joints that the machine cannot make: a wire, a connector that will not survive reflow, a repair or a prototype. The process is manual but it is not uncontrolled, and the iron temperature is the parameter that governs both the joint and the damage.
The temperature has to be high enough to melt the alloy quickly and low enough not to damage the laminate, the component or the plating. The window is narrower than most operators assume, and it moves with the thermal mass of the joint.
Why Temperature Matters
A hot iron transfers energy quickly, so the joint reaches the melting point with a short contact time. A cool iron requires a longer contact, which heats the surrounding laminate and the component body more than a brief contact at a higher temperature.
Total heat input is what damages the board, and it is the product of the temperature and the time. This is why a higher temperature with proper technique is often safer than a low temperature applied for a long time.
Typical Set Points
For a lead free alloy, the iron is usually set between 340 and 380 degrees Celsius at the tip, while a tin lead alloy is worked at a lower setting. The exact value depends on the tip, the joint and the technique.
The set point is the tip temperature with no load, and the tip cools as soon as it touches a joint. The recovery of the station, rather than the set point, is what determines the working temperature on a heavy joint.
Tip Selection and Condition
The tip should match the joint: a chisel for a through hole pad, a fine conical tip for a small pad and a hoof for a drag solder. A tip that is too small for the joint forces a longer dwell and transfers less heat.
A corroded or pitted tip transfers heat poorly and must be replaced rather than filed. Tip condition should be checked at the start of each session, since the difference between a good tip and a worn one is a matter of seconds on every joint.
Thermal Mass and Preheat
A joint connected to a plane takes far more energy than a signal joint, and a small tip cannot supply it from the iron alone. The standard solution is to preheat the assembly from below.
Where preheating is not available, the operator should be given a larger tip and a higher set point for that joint rather than being expected to dwell until the joint flows. The dwell is what damages the board.
Flux and Solder Addition
The flux has to be applied so that it reaches the joint while it is still active, and it should be added before the alloy rather than after. A flux that has already burned contributes nothing and leaves residue.
The alloy should be fed into the joint rather than onto the iron, so that the flux and the alloy work together on the surfaces that are being joined.
Rework and Repeated Contacts
Repeated contact on the same joint accumulates thermal damage in the laminate and can lift a pad. Where a joint needs more than a short correction, the problem is usually insufficient heat rather than technique.
The number of contacts on a joint should be limited, and the joint should be considered for replacement if it has been revisited several times.
Damage That Indicates Excess Heat
A lifted pad, a discoloured laminate, a swollen mask and a component with a cracked body are all signs that the joint received too much energy. The damage is cumulative, so the board may pass inspection and fail later.
The appearance of the joint itself is also informative, since a dull joint with a rough surface indicates either a cold joint or an overheated one. The distinction is made by looking at the pad and the laminate around it.
Operator Qualification
Hand soldering should be qualified with a practical test on representative joints, including one on a plane connected pad. The test should be judged against the standard described for joint workmanship.
The station should have a written set point, a specified tip and a preheat setting for each joint type, so that two operators produce comparable results.
Records
Where hand soldering is used in production, the temperature, the tip and the operator should be recorded for each batch. Where it is used for rework, the record should identify the joint and the reason.
These records belong with the process evidence described for manufacturing processes, since hand soldering is a process step like any other and not an exception to the system.
Process Control and Verification
On a design of this kind, thermal mass is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, thermal mass is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Is a hotter iron always more damaging? No. Total heat input matters, so a short contact at a higher temperature can be gentler than a long contact at a low one.
Should the tip be filed to restore it? It should not, since removing the plating exposes the copper and destroys the tip. A worn tip is replaced.
Why does a plane connected joint need preheat? Because the plane conducts energy away faster than a small tip can supply it, so the joint would otherwise need a long dwell.
Can hand soldering meet the same standard as reflow? It can, when the temperature, the tip, the flux and the technique are controlled and the operator is qualified.
